1. Dress: The absolute first impression you will make on someone will be through your clothing, because that is what is seen from a distance, and cannot change throughout your meeting. Make sure to dress according to the situation-don't over or under dress-and maintain within the limits of good taste. If you aren't sure if what you're wearing looks good, ask people for an honest opinion. One last thought: always, and I mean always, pull up your pants.
2. Hygiene: Take a shower! Shave! Brush your teeth! You must be fully bathed and groomed before you meet with someone for the first time, because scruffy looking people generally don't seem as neat and mature. Pay attention to the little elements like breath: keep a pack of mint gum with you wherever you go, and periodically check to make sure you aren't killing bugs every time you breathe out. If you sweat heavily, keep a small stick of deodorant/anti-perspirant close, and if you notice you're stinking you can freshen up. People notice the minutiae!
3. Manners: At the table and with other people be civilized, polite and respectful: keep your elbows off of the table, open doors for people and address everyone-initially, at least-by their formal title. This will make an especially good impression on senior citizens, because you will prove that you aren't one of those "new fangled punks."
4. Speech: Have clean, clear diction and speak sans "like" or "you know." It is important to be articulate because that inspires a feeling of intelligence and education in the person you are meeting with. Always leave out profanity, and whatever you do, make sure to speak loud enough for all to hear, because conversationalists are easily agitated if you force them say "excuse me?" more than a few times.
5. Discretion: Choose what to share about yourself: forget to tell everyone about that time you went camping and ruptured your appendix, then fell face first into a pile of bug infested leaves-it is rude and will alienate you from the group. Try to withhold from conversations on personal subjects like religion or more disgusting topics like personal medical care. Before you speak, think about the possible impact of what you might say, then imagine its implications in the long run.
6. Humor: Humor can be your most powerful tool or your doom, because everyone has a slightly different sense of humor. What might be hilarious to you might seem disgusting to another, or vice versa. Try to withhold from any jokes that aren't family or dinner table friendly; you can tell those later.
7. Start and End with a Bang: I am a classical musician, and in my orchestra, among other messages, the conductor tells us that the "audience remembers mostly the first and last notes of a symphony." This is the same in a personal encounter: whoever you are meeting with will remember how you greet them, and then in what manner you left them. If you feel you have trouble with this, practice a few different phrases in the mirror, and introduce elements like: "pleased to meet you," or "honored to make your acquaintance." Ignore the antiquity of these phrases; it often makes them more memorable.
Making a good impression will set any relationship off on a good foot. If you are in a situation where you need to be judged at face value-such as a job interview or date-then make sure to go through this list and make sure you are within bounds of reason and good taste on all of your decisions.